One quality control issue in manufacturing processes is making accurate quality assessments of the geometry of nominally circular symmetric parts. Geometrically accurate rounded parts are necessary components of rotating machinery such as turbines used in power generation or propulsion. Cumbustors, turbine casings, and the like have geometric shapes ranging from straight cylinders to complex circularly symmetric parts of irregular axial cross section. For quality control and assessment, it is required in the fabrication process to accurately measure part diameter (and whether the part conforms to the desired geometric shape) at any of a number of axial positions, and further to determine whether the part is concentric with the spin axis of a machine tool rotary stage.
Diameter measurements are commonly made with calibrated calipers that encircle the part (or workpiece) so as to come into contact with opposite side surfaces of the part. This process is often used even with large parts (such as a turbine rotor) and presents potential difficulties with respect to accuracy. For example, such a process is very dependent on the skill of the person making the measurement due to, for example, the flexing of the instrument when it comes in contact with the part being measured. Further, inaccuracies can arise because the measurement is made by the calipers at only one or a few angular positions around the part. Other measurement devices similarly require contact with the surface of the part to be measured, such as calibrated wheels that are held in contact with the workpiece as it is rotated through one revolution. Knowledge of the diameter of the calibrated wheel and the number of rotations enables one to determine the circumference and thence the diameter of the workpiece.
Another problem area is the potential deformation or damage to the part from measurement activities. Contact with the part (e.g., by either a calibrated-wheel type of device or a caliper) can lead to such deformation and damage, as can handling of the part to obtain the measurement with such contact devices.
For these reasons, among others, it is desirable to have a measurement system that can precisely measure the diameter of curved parts without the necessity of contacting the part.